Rotary precoat filters

For the most economical operation of a rotary precoat filter, a number of factors must be considered ... 

There are three common types of filters rotary precoat filters, cartridge filters, and stacked paper plates. In practice, paper plates are best. In particular, Sparkler stacked filters are easy to maintain. Operation of rotary precoat fillers is too complex for many refinery applications. Cartridge filters are good, except that the cartridge cost can be high when frequent change-outs are necessary. Stacked paper plate filters are quite simple The paper is discarded after each use. On one unit it was estimated that 1,000 lb of particulates had accumulated in one 50-plate Sparkler filter. 

In the precoat mode, filter aids allow filtration of very fine or compressible soHds from suspensions of 5% or lower soHds concentration on a rotary dmm precoat filter. This modification of the rotary dmm vacuum filter uses an advancing knife continuously to skim off the separated soHds and the... 

In precoating, the prime objective is to prevent the filter medium from fouling. The volume of initial precoat normally applied should be 25 to 50 times greater than that necessary to fill the filter and connecting lines. This amounts to about 5-10 lb/100 fF of filter area, which typically results in a 1/16-in. to 1/8-in. precoat layer over the outer surface of the filter medium. An exception to this rule is in the precoating of continuous rotary drum filters where a 2-in. to 4-in. cake is deposited before filtration. The recommended application method is to mix the precoat material with clear liquor (which may consist of a portion of the filtrate). This mixture should be recycled until all the precoat has been deposited onto the filter medium. The... 

A mixture of the hydrolyzate with diatomaceous earth, added as a filter aid, is then filtered on rotary, vacuum, precoated filters. The filtrate is a clear, slightly yellow, sweet-tasting liquor containing about 0.1% of protein and 0.2% of ash. The rest is carbohydrate, made up of 95.0% of D-glucose, 2.0% of maltose, 1.6% of isomaltose, and 1.4% of tri- and higher saccharides. 

There are two types (a) the vacuum leaf filter and (b) the rotary vacuum precoat filter (Figs. 16 and 17). 

There are some liquids with solids so slimy that it is impossible to filter them on a pressure or vacuum leaf filter. Other liquids contain such a high volume of solids that the filters described previously would quickly become filled with cake, resulting in an uneconomical amount of time spent in cleaning and precoating. The rotary vacuum precoat filter was designed to meet these conditions. 

Doctor Blade (knife) A sharp, hard blade that cuts the cake off the surface of a filter. Usually found on rotary vacuum precoat filters. 

RVPF (Rotary Vacuum Precoat Filter) A drum filter that is coated with a thick (up to 6 in.) precoat of filter aid, such as CELITE or equivalent. 

With the exception of the precoat applications, RVF s do not usually yield absolutely clear filtrate. Although still widely used, rotary vacuum filters are, in some cases, being replaced by membrane separation technology as the method of choice for clarification of fermentation broths and concentrating cell mass. Membranes can yield more complete filtration clarification, but often a wetter cell paste. 

Although rotary drum filters are normally employed for materials of moderate to low resistance, they can be used as precoat filters for highly resistant slurries. A thick coat of several centimeters of filter aid is deposited on the drum prior... 

Fig. 20.9 A beer and yeast recovery system. 1. Fermenter 2. Beer clarifier 3. Surge tank 4. Yeast suspension tank 5. Filter aid mixer 6. Yeast (autolysis) tank 7. Rotary vacuum filter 8. Top feed applicator 9. Precoat applicator 10. Yeast suspension spray 11. Defoamer 12. Vacuum pump 13. Beer recovery tank 14. Pasteurizer 15. Yeast drier A. Wort preparation B. Beer treatment C. Dried yeast. (Based on drawing of Alfa-Laval Ltd.)...

Hofiinger, W. and Hacki, A, 1990, Evaluation of optimal knife advances for precoat filtration on rotary vacuum filters, Filtrat. and Separat, 27, pp 110-113. 

Figure 1.43 Schematic representation of the precoat rotary drum filter.

Filter aids can improve the permeability and sometimes porosity of a filter cake, improve filtrate clarity and help to prevent filter medium blinding. They comprise relatively porous particles such as diatomite, perlite and activated carbon and are either filtered as a precoat onto the medium or mixed as body feed with the suspension during a pretreatment stage the latter beneficially improves the porosity of a subsequently formed filter cake. Both the cost of filter aid and the need to remove filter aid from the processed solids can present problems however, the use of filter aids on rotary drum filters and in the filtration of dilute feeds (such as those found in the brewing industries) can bring undoubted benefits. More detail on filter aids is provided in Chapter 2. 

Milled perlite (filter aid) - Some expanded perlite is milled to -100 mesh and sold as a filteraid, especially for use in rotary precoat filtration. Careful milling is required, because oversize particles will inhibit the formation of filtercake while excessive fines will slow the filtration rate. 

Rotary-type enclosed continuous filters are used Almost exclusively in solvent dewaxing because the solvents are valuable and because they usually, form explosive mixtures with air. The principle of operation is the same as that outlined on page 321 with regard to the Oliver precoat filter. However, a precoat is not employed in wax filtering, and the rotating drum is enclosed in a shell so that pressure can be applied (Fig. 12-4). Some plants employ vacuum rather than pressure. The wax cake is sprayed or washed with solvent while it is still on the drum. Closed filtering systems are required because the solvents must be... 

A rotary vacuum drum precoat filter is a specially designed version of the vacuum drum designed to use a thick layer of precoat material to clarify liquid flows. Initially a slurry of the precoat material is prepared and filtered to produce a cake of up to 100mm thickness on the filter cloth. Then the feed trough is drained and refilled with the suspension to be filtered, which is now filtered in the normal way through the precoat layer. The filtered cake can be washed if necessary, and is then discharged by means of a full-width scraper blade. The blade removes the whole of the cake, together with a thin slice (about 0.5 mm) of the precoat layer. 

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